Research On Terahertz Metasurfaces With Tunable Absorption And Polarization Characteristics

Terahertz（THz）waves,with frequencies ranging from 0.1 to 10 THz,are electromagnetic waves that hold broad prospects for applications in various fields such as communication,biomedicine,sensing,and imaging.However,the weak interaction between THz waves and natural materials has limited the development of THz technology.To overcome this challenge,researchers have turned to artificially designed metasurface structures.Metasurfaces offer advantages such as miniaturization,ultra-thinness,ease of integration,and mature fabrication processes.The combination of THz waves and metasurfaces is considered the optimal solution to overcome the limitations in THz technology development,leading to the development of various metasurface devices such as beam splitters,biosensors,filters,polarization converters,and absorbers.Despite extensive research on the absorption and polarization characteristics of THz metasurfaces,issues such as single functionality,inflexible modulation,and poor functional integration persist.This paper develops THz metasurface devices using graphene,vanadium dioxide（VO₂）,and photosensitive silicon as tuning materials.The absorption and polarization characteristics of the devices are studied theoretically using the finite-difference time-domain（Finite Difference Time Domain,FDTD）method and the finite integration technique（Finite Integration Technique,FIT）.The results are analyzed using transmission line theory,impedance matching theory,interference theory,and other methods.The specific work is as follows:1.Development of a multifunctional THz metasurface absorber based on graphene and VO₂,which achieves interconversion between single-band and dual-band absorption using the phase transition properties of VO₂.Dynamic control of absorption is achieved by changing the Fermi energy level（E_f）of graphene and the conductivity of VO₂.The absorption characteristics and dynamic modulation mechanism are studied using the FDTD method and impedance matching theory.2.Design of a THz absorber with switchable single-band and three narrowband absorption functions.It is found that when VO₂ is in the insulating phase,the structure exhibits single-band absorption in the range of 0.8-2.4 THz.The absorption rate can be adjusted from 20%to 95%by changing the E_fof graphene.When VO₂ transitions to the metallic phase,the metasurface exhibits three narrowband absorptions.The physical mechanisms of absorption and absorption modulation are explained using impedance matching theory and transmission line theory.3.Study of a multifunctional THz metasurface capable of polarization conversion and absorption.When VO₂ is in the insulating phase,the metasurface demonstrates over 90%polarization conversion efficiency and significant asymmetric transmission characteristics.Flexible control of dual-band transmittance is achieved by adjusting the conductivity of photosensitive silicon.When VO₂ transitions to the metallic phase,the metasurface exhibits bidirectional THz wave absorption characteristics.Additionally,the intensity and frequency of absorption can be adjusted by changing the conductivity of photosensitive silicon.Preliminary exploration of the application of this structure in near-field imaging is conducted.4.Proposal of a chiral THz metasurface capable of achieving absorption characteristics, circular dichroism（CD）,and linear-circular polarization conversion.Dynamic tuning of CD is achieved by adjusting the conductivity of photosensitive silicon,and dynamic control of absorption characteristics is achieved by changing the E_f of graphene.Based on the Pancharatnam-Berry phase effect of this chiral structure,dynamically controllable anomalous reflection,vortex beam,and focusing functions are realized.These studies provide insights into the development of THz metasurfaces and lay a theoretical and experimental foundation for the design and application of future high-performance electromagnetic wave devices.